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Washington – US president-elect Donald Trump on Friday tweeted that Mexico will reimburse American taxpayers for a new border wall and that US money spent will be for the "sake of speed".

His tweet came as top aides consider a plan to ask Congress to ensure money is available in US coffers for the wall, but to rely on existing law that already authorises fencing and other technology along the southern border.

The potential approach was confirmed on Thursday by two congressional officials and a senior transition official with knowledge of the discussions; all spoke on condition of anonymity because they were not authorised to discuss the matter publicly.

Trump said in a tweet early Friday: "The dishonest media does not report that any money spent on building the Great Wall (for sake of speed), will be paid back by Mexico later!"

Mexico's president and other senior officials have repeatedly insisted that Mexico won't pay for a wall.

Legislative fight

During his campaign, Trump repeatedly told voters if elected he would build a wall along the US southern border and make Mexico pay for it.

The approach could also stave off a legislative fight that Trump might lose if he tried to get Congress to pass a measure authorising the kind of border wall he promised during the campaign.

It's not clear how much could be done along the 3 200km border without additional actions by Congress.

Lawmakers passed the Secure Fence Act of 2006, but most of those 1 100km have already been built. Some areas are in much better shape than others, though, and long stretches are made up of fencing that stops vehicles but not pedestrians.

But whatever steps might be taken without Congress' approval would be likely to fall short of the extravagant new wall on the border that Trump repeatedly said Mexico would pay for during his campaign for the White House.

And despite Congress' involvement in approving any spending, such an approach might also open Trump to charges of circumventing the House and the Senate to take unilateral actions, something he repeatedly criticised US President Barack Obama for doing.

A spending bill including money for border construction could also provoke a legislative showdown given potential opposition from Senate Democrats.

Still, several lawmakers and congressional officials said the administration could have significant flexibility in taking additional steps without Congress' approval.